1. Field of the Invention
The present invention relates to a method of driving a liquid-drop-ejecting recording head which carries out recording of an image by ejecting liquid drops.
2. Description of the Related Art
As conventional liquid-drop-ejecting recording heads, there are inkjet recording heads which record an image by ejecting ink drops onto a recording sheet, and the like.
For example, in an inkjet recording head which carries out gradation expression by adjusting the amount of ink forming one pixel, a driving waveform corresponding to the density is selected and applied to a piezoelectric element, so as to adjust the amount of ink which is jetted-out.
In the case of using, as the driving waveform, an analog driving waveform in which the voltage amplitude and the time change are arbitrary, the range over which the dot diameter can be modulated can be made to be large, and it is also easy to jet-out the desired amount of ink. However, there are the problems that, due to an analog amplifier and other driving circuits, the driving circuits are large and a large amount of electric power is consumed.
Thus, in recent years, making the driving circuit more compact and less expensive and decreasing the amount of consumed electric power by using a binary digital driving waveform have been studied. In a binary digital driving waveform, the voltage amplitude is determined by the power source voltage, and the time change is determined by a time constant depending on the electrostatic capacitance C of a piezoelectric element and the on resistance R of a switching element. Therefore, the degrees of freedom of the driving waveform are greatly reduced, and it is very difficult to make to the range of modulation of the dot diameter large and to adjust the ink amount to the desired ink amount.
As a conventional technique for adjusting the ink amount in a wide range by using a digital driving waveform, Japanese Patent Application Laid-Open (JP-A) No. 11-170515 discloses a technique of adjusting the interval of a jetting pulse and a non-jetting pulse, so as to vary the ink drop volume while maintaining the ink drop speed.
Further, JP-A No. 11-170522 discloses a technique of changing the ink drop volume by changing the repetition frequency of the jetting pulse.
JP-A No. 2002-326357 discloses a technique of applying a main pulse after applying a supplementary pulse, and adjusting between these pulses so as to adjust the ink drop volume.
Moreover, JP-A No. 10-151770 discloses controlling the ink drop volume of a small drop region by voltage, and controlling the ink drop volume of a large drop region by the number of pulses.
However, in each of the techniques disclosed in the aforementioned JP-A Nos. 11-170515, 11-170522, and 2002-326357, there are the problems that the range of adjustment of the ink drop volume is narrow, and gradation expression in the high density range in particular cannot be carried out well. Further, the control is complex in the case of changing the repeat frequency as in JP-A No. 11-170522.
Moreover, the technique disclosed in JP-A No. 10-151770 has the problems that, because a circuit for carrying out voltage control is needed, the driving circuit becomes large and complex.
The technique disclosed in JP-A No. 10-151770 also has the problems that, when the number of pulses increases, the driving waveform becomes long, and the printing frequency decreases.